Well, I think the thought is that most don't want white. But you can change them to white in WindowBlinds config under the Modify Theme button.
I have tried the "modify style" - change system colour options. Does not work correctly on many skins, and a few skins get totally messed up colourwise, so I gave that up.
And btw, I asked around at work, and nobody wanted anything but white as a background for daily use, so I can't be all alone.